1. Field of the Invention
The present invention relates to a gas turbine, a heavy-duty gas turbine blade, which has horizontally extending protrusions, and a manufacturing method for the gas turbine blade.
2. Description of the Prior Art
Primarily Ni-base superalloys have been used as materials for the rotor blades of electricity generating gas turbines. To improve the thermal efficiency of gas turbines, the temperature of gas has been increased year after year. To cope with such an increase in the gas temperature, conventional casting blades having complicated cooling holes therein have been employed.
Single-crystal vanes have already been used as rotor blades of aircraft jet engines. Alloys for casting the single-crystal vane are developed on the assumption that they do not have grain boundaries, and therefore they do not contain grain boundary strengthening elements such as B, Zr and Hf. For this reason, the grain boundaries of single-crystal alloys are weak. At least a portion of a casting must be single-crystallized before the casting can be used. In order to use the single-crystal vane as a gas turbine rotor blade, it is indispensable for the entire casting to be single-crystallized.
Most single-crystal castings are manufactured by a unidirectional solidification process disclosed in Japanese Patent Laid-Open Nos. 51-41851 and 1-26796. This process is a process in which a casting is withdrawn downwardly from a heated furnace and is solidified gradually from the lower end to the upper end thereof.
The rotor blade for the aircraft jet engine has a length of approximately 10 cm, and the cross-section area of a shaft is 10 cm.sup.2 at the largest. The size of a platform extending horizontally from the main body of the rotor blade is small. Because the entire rotor blade is such a small component, a single-crystal vane can be manufactured by solidifying a vane-shaped casting through the above unidirectional solidification process.
However, rotor blades in electricity generating gas turbines are larger than those in aircraft jet engines. The former have a length of 14-16 cm or more and shanks having a cross-section area of 15 cm.sup.2 or more. It is therefore difficult to manufacture the former in a single-crystal structure. There are portions, such as the platform and sealing portions extending from the side of the shank, protruding horizontally from the direction in which the casting is solidified. Even when the casting is solidified by the conventional unidirectional solidification process, the entire casting cannot be single-crystallized. The following reason may be attributed to the non-single crystallization. When the casting is solidified, the horizontally protruding portion begins to solidify from the outer periphery of the casting. Since the horizontally protruding portion has no relationship with the other portion of the casting, it will have a crystal orientation different from that of the other portion. When this portion and the other portion of the casting are further solidified and the crystals of both come into contact with each other, the contacting surface is formed into a grain boundary, thus preventing a single crystal from growing.
It is thus impossible to form an entire large turbine blade for use in an electricity generating gas turbine in a single-crystal structure.